1. Field of the Invention
The present invention relates to a strain sensing device such as an acceleration sensor or a pressure sensor.
2. Description of the Related Art
In a usual acceleration sensor, a weight displaces upon acceleration. A stopper is provided in the direction of that displacement in the range of possible displacement of the weight. Even with large movement of the weight due to excessive acceleration, the stopper ensures that the displacement can be stopped, thereby preventing breakage of the strain generating portion, which is made of thin material.
At the present time, however, this stopper is attached to the sensor portion at a final stage of its manufacture, and no consideration has been given at all to prevention of breakage of the thin portion due to vibration etc. acting on the weight during the process of manufacture of the thin portion of the sensor.
Note in this regard that in order to enable the acceleration sensor to detect minute acceleration, the beam on which the strain gauge device is placed is generally made thin. This enables the amount of displacement of the weight, which displaces along with acceleration, to be detected with a high sensitivity. The same is true in a pressure sensor. To increase the sensitivity, the general practice is to make the diaphragm thinner.
In other words, to obtain higher sensitivity in such strain sensing devices, the beam, diaphragm, and other strain generating portions need only be made thinner. This thinning, however, ends up causing a reduction in the mechanical strength vis-a-vis vibration or excess stress.
In recent years, demand has been growing for making the sensor portion of such strain sensing devices and the IC circuit portion for processing the output signals from the sensor portion on the same chip so as to achieve superminiaturized strain sensing devices. Along with this reduction in size of the devices, the strain sensing portions have to be made increasingly thinner, which will further aggravate the problem of the reduction of strength at such portions. In particular, it is expected that there will be remarkably more breakage of the thin portions due to vibration etc. during the process of manufacture.
Therefore, the present inventors thought of fabricating a supersmall type strain sensing device by a film construction using conventional IC processes and took note of the construction of the stopper in the process of manufacture of the sensing devices.
In this regard, note that when a sensor portion and IC circuit portion are made on a single chip, the sensor portion, i.e., the strain sensing device, has a movable strain sensing portion and therefore requires that a cavity be formed in the sensor portion during the process of its manufacture.
In conventional IC processes, when a cavity is formed in a substrate, contaminants, dust, chemicals, etc. tend to accumulate in the cavity. When high temperature processes are required, in particular, these contaminants etc. adhering to the substrate can cause contamination of the manufacturing apparatuses and can spread in the substrate to cause deterioration of the properties of the IC portion.
The present invention was made in consideration of all of the above and has as its object the provision of a supersmall strain sensing device which offers a high sensitivity and which is free from breakage of the strain generating portion and, further, the provision of a method of manufacture enabling the strain sensing device to be manufactured with a good precision without breakage of the strain generating portion during the process of manufacturer and without being affected by contaminants etc.